Robot and method of controlling the robot

ABSTRACT

A robot capable of being operated at a high-speed by the full use of a power of a servo-motor, including a motor and a speed reducer, wherein the motor drives the robot through the speed reducer, and the speed reducer is a variable speed reducer capable of varying a reduction ration thereof while the robot is operating reproducibly.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a robot that is driven by a motor via aspeed reducer, and in particular, a vertical multi-joint robot.

2. Description of the Related Arts

Nowadays, the mainstream of so-called vertical multi-joint robots forindustrial uses resides in electric robots that are driven byservomotors. Generally, because servomotors have a higher rotation speedand a lower torque than the rotation speed and torque which arerequested for robots, a speed reducer intervenes between a robot and aservomotor. Although a cyclone speed reducer and a harmonic speedreducer are adopted as such a speed reducer, the reduction ratio isfixed in either case, and the output of the servo motor is reduced at aconstant reduction ratio.

As an exceptional example, an electric robot is disclosed by JapaneseUnexamined Patent Publication No. Sho-64-51285, which is provided withmeans for varying the reduction ratio between a joint mechanism and thespeed reducer. However, in a robot of the above-described invention,that is, in a direct teaching robot in which teaching is carried out byan operator operating the robot with its arms, the reduction ratio ischanged in teaching and in playback operations, wherein the reductionratio in the playback operation is fixed at a constant value.

However, in this type of conventional vertical multi-joint robot, areduction ratio suited from such a maximum load is determined on thebasis of a load status applied to the servomotor and is maximized withthe design maximum load mass attached onto the robot, that is, a statuswhere the rotational inertia is maximized when the arm is swungextremely outside in the case of swinging on the horizontal surface, anda status where a moment resulting from gravity with the arm horizontallyshifted down in the case of swinging of a forward and backward swingingaxis.

Accordingly, where the robot is used with a load mass not reaching themaximum load mass, or where the robot is used in a state where themoment due to a rotational inertia and gravity is smaller than themaximum, a problem occurs in that a sufficient operation speed cannot beobtained while the power of the servomotor becomes excessive. That is, aproblem occurs in that a speed, which could be obtained if a reductionratio responsive to a load is selected, cannot be obtained.

SUMMARY OF THE INVENTION

Therefore, it is an object of the present invention to provide a robotcapable of operating at a high speed by fully utilizing the power of aservomotor.

In order to solve the above-described and other problems, a robotaccording to a first aspect of the invention, which is driven by a motorvia a speed reduce which is made into a speed-varying reducer whosereduction ratio is varied during a playback operation of theabove-described robot. The robot according to a second aspect of theinvention varies the reduction ratio of the above-described speedreducer in response to the degree of an angle of the arm of the robot.The robot according to the third aspect of the invention varies thereduction ratio of the above-described speed reducer in response to thesize of a load mass attached to the tip end of the arm of theabove-described robot. The robot according to the fourth aspect of theinvention varies the reduction ratio of the above-described speedreducer in response to the intensity of a rotational inertia around thedrive shaft of the above-described robot. Also, a method for controllinga robot, which is driven by a motor via a variable speed reducer whosereduction ratio is variable during a playback motion, according to thesixth aspect of the invention comprises the steps of teaching the robotappointed operation courses; acquiring fluctuations in a rotationalinertia around the drive shaft of the above-described robot and in anangle of the above-described robot arm; and determining a schedule forvarying the above above-described speed reducer by the fluctuations in aload of the above-described motor resulting from the fluctuations in theabove-described rotational inertia and angle of the above-described arm.The robot according to the seventh aspect of the invention comprises thesteps of teaching the above-described robot appointed operation coursesby an off-line teaching device to acquire fluctuations in the rotationalinertia around the drive shaft of the above-described robot and angle ofthe above-described robot arm by simulation brought about by theabove-described off-line teaching device; and determining a schedule forvarying the reduction ratio of the above-described speed reducer incompliance with the fluctuations in the load of the above-describedmotor resulting from the above-described rotational inertia and angle ofthe above-described arm.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a conceptual view of a vertical multi-joint robot showing apreferred embodiment of the invention;

FIGS. 2(a), 2(b) and 2(c) are views describing a vertical multi-pointrobot showing the preferred embodiment of the invention, wherein FIG.2(a) shows a status where the lower arm 3 is erected and the upper arm 4is made horizontal, FIG. 2(b) shows a status where the lower arm 3 isshifted down forward from the erected status by an angle θ, and FIG.2(c) shows a status where both the lower arm 3 and the upper arm 4 areerected.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, a description is given of a preferred embodiment of theinvention on the basis of the accompanying drawings.

FIG. 1 is a conceptual view of a vertical multi-joint robot. In thedrawing, 1 denotes a fixed base. A swing head 2 is attached to the fixedbase 1 so as to freely swing around a swing axis (S axis). The lower arm3 is attached to the upper portion of the swing head 2 so as to freelyswing forward and backward around a forward and backward swinging axis(L axis). The upper arm 4 is attached to the upper portion of the lowerarm 3 so as to freely swing around a vertical swinging axis (U axis). 5denote a load mass that is attached to the tip end of the upper arm 4.Herein, the load mass 5 corresponds to a mass of an end effecter, whichis attached to the tip end of the robot.

6 denotes a speed reducer attached to the swing head 2. The outputportion thereof is coupled to the fixed based 1, and the input portionthereof is fixed at a servomotor 7. The swing head 2 is driven andswiveled around the S ahaft by the servomotor 7 and speed reducer 6.

8 denotes the first servomotor with a speed reducer, which is fixed onthe swing head 2, and the output portion thereof is coupled to the lowerarm 3 and drives and swings the lower arm 3 around the L axis.

9 denotes the second servomotor with a speed reducer, which is fixed onthe lower arm 3, and the output portion thereof is coupled to the upperarm 4, and drives and swings the upper arm 4 around the L axis.

The speed reducer 6, the first servomotor 7 with a speed reducer, andthe second servomotor 8 with a speed reducer are provided with amechanism by which the reduction ratio is changed over to two ratios(high and low) during playback of the robot by an instruction of a robotcontrolling apparatus (not illustrated). The mechanism for changing thereduction ratios may be selected among publicly known mechanisms, forexample, a mechanism in which engagements of gears are changed, amechanism in which two sets of planet gears are combined, and amechanism in which engagements of pulleys are changed.

Here, a description is given of actions of the robot. FIGS. 2(a), 2(b)and 2(c) are views describing three types of statuses of the robot,wherein FIG. 2(a) shows a status where the lower arm 3 is erected andthe upper arm 4 is made horizontal, FIG. 2(b) shows a status where thelower arm 3 is shifted down forward from the erected status by an angleθ, and FIG. 2(c) shows a status where both the lower arm 3 and the upperarm 4 are erected.

The distance from the S axis to the center of gravity of the load mass 5is X1 in the status shown in FIG. 2(a). However, where the lower arm 3is shifted down as in FIG. 2(b), the distance becomes large to be X′1.Also, the centers of gravity of the lower arm 3 and the upper arm 4 aremade farther from the S axis. Therefore, in the status shown in FIG.2(b), the rotational inertia around the S axis becomes larger than inthe status shown in FIG. 2(a).

To the contrary, in the status shown in FIG. 2(c), the distance from theS axis to the center of gravity of the load mass 5 becomes small asshown with X1″, and the centers of gravity of the lower arm 3 and upperarm 4 approaches the S axis, wherein the rotational inertia around the Saxis is decreased.

As the rotational inertia around the S axis increases, the load of theservomotor 7 is accordingly increased in speed acceleration anddeceleration. As the rotational inertia around the S axis decreases, theload is decreased. Therefore, the reduction ratio of the speed reducer 6is varied in response to the statuses of the lower arm 3 and upper arm 4by instructions of the robot controlling apparatus. That is, where theangles of the lower arm 3 and the upper arm 4 are in a range where therotational inertia around the S axis exceeds an appointed value, alarger reduction ratio is selected, and in a range where the rotationalinertia around the S axis becomes smaller than the appointed value, asmaller reduction ratio is selected.

Further, the rotational inertia around the S axis is calculated whiletaking the weight of the lower arm 3 and the upper arm 4 intoconsideration, and the reduction ratio may be selected in compliancewith the size of the result.

In addition, the reduction ratio maybe selected or determined incompliance with the size of the load mass 5. Generally, the larger arobot becomes, the larger the maximum mass, that is, the conveyable massand range of movement of an attachable end effecter become. The maximumspeed and maximum acceleration of the robot then tends to becomesmaller. There are cases where a large-sized robot having a largerconveyable mass in comparison with the actual mass of the end effecteris selected in order to secure a larger range of movement. In suchcases, if a smaller reduction ratio in comparison with the mass of theend effecter is selected, a large-sized robot can be operated at a highspeed.

Next, a description is given of alternation of the reduction ratio ofthe first servomotor with a speed reducer, which drives the lower arm 3.

The distance from the center of rotation of the L axis to the center ofgravity of the load mass 5 in the status shown in FIG. 2(a), that is,the length X2 of a lever of a gravity moment loaded onto the firstservomotor 8 with a speed reducer by the load mass becomes long so as tobe X2′ in the status shown in FIG. 2(b). Also, the length of theabove-described lever becomes 0 in the status shown in FIG. 2(c).

The moment of a gravity loaded onto the first servomotor with a speedreducer by the load mass 5 is thus maximized when the statuses of thelower arm 3 and upper arm 4 are horizontal, and becomes 0 when both ofthe statuses of the lower arm 3 and the upper arm 4 are erect.

Therefore, an instruction for varying the reduction ratio of the firstservomotor 8 with a speed reducer is issued to the robot controllingapparatus in response to the angle with respect to the statuses of thelower arm 3 and the upper arm 4, that is, reference status (usually,either one of the horizontal status or erect status may be used as thereference status).

Also, the moment of gravity loaded onto the first servomotor 8 with aspeed reducer is calculated on the basis of the statuses of the lowerarm 3 and the upper arm 4, and the reduction ratio may be varied inresponse to the size of the result thereof.

As in the above, the reduction ratio of the second servomotor 9 with aspeed reducer, which drives the upper arm 4, may be varied.

In the status shown in FIG. 2(a), the length of a lever of gravityloaded onto the second servomotor 8 with a speed reducer by the loadmass 5 is expressed in terms of X2. However, in the status shown in FIG.2(c), the length of the above-described lever becomes 0.

The moment of gravity loaded onto the second servomotor 9 with a speedreducer by the load mass 5 is maximized when the status of the upper arm4 is horizontal, and becomes 0 when the upper arm 4 is erect.

Therefore an instruction for varying the reduction ratio of the secondservomotor 9 with a speed reducer in response to the angle with respectto the status of the upper arm 4, that is, reference status (usually,either one of the horizontal status or erect status may be used as thereference) is issued to the robot controlling apparatus.

In addition, in a case where the moment of gravity loaded onto the motorvaries at the beginning point of movement of the robot and at thetermination point thereof, the reduction ratio may be selected on thebasis of a larger load. As a matter of course, an intermediate point isdefined between the beginning point and the termination point, whereinthe robot may be once stopped at the intermediate point to change thereduction ratio. However, this is not necessarily advantageous in viewof an increase in time required to change the reduction ratio and a lossof time resulting from speed deceleration and acceleration.

The above-described method for varying the reduction ratio is based onthe statuses of the respective arms of a vertical multi-joint robot, andis based on a static force balance. However, because a plurality of axesare simultaneously operated in an actual robot, torque resulting frominterference of respective axes is generated, wherein there is a casewhere the load of the drive motor cannot be obtained only by the staticforce balance.

Therefore, a description is given of a method for determining a schedulefor varying the reduction ration of a speed reducer based on afluctuation Of the load of the motor by actually operating the robot andobtaining the fluctuation in the load of the motor.

(1) First, an appointed operation course the robot (For example, acourse passing through respective teaching points from A to F likeA→B→C→D→E→F).

(2) The reduction ratios of the respective axes are set to largervalues, and the robot is operated for playback in compliance with thecourse previously taught to the robot. At this time, the speed, torqueand current of the motor are recorded.

(3) A schedule for varying the reduction ratio is determined incompliance with a fluctuation in the torque of the motor, which haspreviously been recorded. For example, when the torque of the motorfluctuates such as Large→Large→Small→Small→Large→Small in the coursefrom A to F, a schedule for changing the reduction ratio at therespective teaching points from A to F in order ofLarge→Large→Small→Large→Large→Small is programmed.

(4) The robot is played back based on the program in which the schedulefor changing the reduction ratio is incorporated, and the speed, torqueand current of the motor are recorded.

(5) Unless the speed, torque and current of the motor exceed the limits,the process is terminated. If any one of these exceeds the limit, stepsfrom (3) through (5) are repeated.

Also, if the steps from (1) through (5) are executed in simulation in acomputer, not dependent upon an actual robot, that is, in so-calledoff-line teaching software, there is no case where the robot overruns orthe motor is not burned out even if the speed, torque and current of themotor exceed the ratings during the playback. Steps (1) through (5) canbe repeated until the schedule for changing the reduction ratio isconverged to the optimal.

Further, in the above-described embodiment, a two-stage variable speedreducer is employed, in which a large reduction ratio and a smallreduction ratio can be selected in the speed reducer. However, thepresent invention is not limited to the two-stage variable speedreducer, wherein it is needless to say that three or more stages of thevariable speed reducer may be employed or a valiable-free speed reducermay be also employed.

As described above, according to the invention, since the reductionratio of the speed reducer can be optimally varied in response to a loadof the respective drive shaft based on the status of the robot and thesize of a mass of the end effecter, power of the drive motor can befully brought about. Therefore, the robot can be operated at a highspeed.

The present invention is effective as a robot that is driven by anelectric motor via a speed reducer, in particular as a verticalmulti-joint robot, and as a method for controlling the same.

What is claimed is:
 1. A robot driven by a motor via a speed reducer,wherein said speed reducer is a variable speed reducer whose reductionratio is variable during a playback motion of said robot, and thereduction ratio of said speed reducer is varied according to the angleof said robot arm.
 2. A robot driven by a motor via a speed reducer,wherein said speed reducer is a variable speed reducer whose reductionratio is variable during a playback motion of said robot, and thereduction ratio of said speed reducer is varied according to the size ofa load mass attached to the tip end of said robot arm.
 3. A robot drivenby a motor via a speed reducer, wherein said speed reducer is a variablespeed reducer whose reduction ratio is variable during a playback motionof said robot, and the reduction ratio of said speed reducer is variedaccording to the intensity of a rotational inertia around the driveshaft of said robot.
 4. A method for controlling a robot, driven by amotor via a variable speed reducer whose reduction ratio is variableduring a playback motion, comprising the steps of: teaching appointedoperation courses to acquire fluctuations in a rotational inertia aroundthe drive shaft of said robot and in an angle of said robot arm, anddetermining a schedule for varying said speed reducer which isdetermined by the fluctuations in a load of said motor resulting fromthe fluctuations in the rotational inertia and the angle of said arm. 5.A method for controlling a robot, comprising the steps of: teaching saidrobot appointed operation courses by an off-line teaching device;acquiring fluctuations in the rotational inertia around the drive shaftof said robot and angle of said robot arm by simulation brought about bysaid off-line teaching device; and determining a schedule for varying areduction ratio of a speed reducer in compliance with the fluctuationsin the load of said motor resulting from the rotational inertia and theangle of said arm.